A semiconductor light receiving device for use in an optical fiber communication (hereinafter referred to as  “optical communication ”) is a device to convert a light signal transmitted through an optical fiber into an electric signal. In a long-range transmission through principal lines, a long wavelength band light beam such as a 1.55 micron band or 1.3 micron band is utilized currently.
In an optical communication, a 1(one) micron level band including a 1.55 or 1.3 micron band is classified as a long wavelength, while a 0.8 micron level band raging from 0.8 to 0.9 micron is classified as a short wavelength.
For convenience, wavelengths are classified in accordance with a classificatory criterion (border line) of about 0.9 micron almost in the same way as classification adopted in an optical communication.
InGaAs/InP based material is used for such a semiconductor light receiving device. InGaAs/InP based material is adaptive for along wavelength band light beam.
An InGaAs/InP semiconductor light receiving device is disclosed in U.S. Pat. No. 6,521,968. The device has a laminated structure of an n-type InP buffer layer formed on an n-type InP substrate, an InGaAs light absorbing layer formed on the buffer layer, and an n-type InP cap layer formed on the light absorbing layer. In fabrication, these layers are laminated on the n-type InP substrate in the above order. A p-type layer is formed by diffusing p-type dopants such as Zn selectively into an region to be a light receiving portion of the n-type InP cap layer. A protecting layer such as SiN is formed on the n-type InP cap layer. A light reflection preventing layer such as SiN is formed on the p-type layer.
A ring-like electrode is formed on a peripheral portion of the p-type layer. Another electrode is formed on a lower surface of the n-type InP substrate.
In the semiconductor light receiving device, incident light signal is absorbed in a depletion layer when a reverse bias voltage is applied to a pn junction of the InGaAs light absorbing layer and the p-type layer. As a result, Electrons and holes are generated in the depletion layer so that a light current is detected by the drift of the Electrons and holes caused by an electrical field. The band gap of the n-type InP cap layer is larger than that of the n-type InGaAs light absorbing layer. This is for the purpose of preventing the contributory ratio of minority carriers generated in the depletion layer to the light current from decreasing by the influence of surface recombination of the minority carriers. A portion of the n-type InP cap layer is turned into p-type to form the p-type layer
Thus, the semiconductor light receiving device operates by a light beam having a wavelength longer than approximately 0.92 micron which is restricted by the band gap of InP, and shorter than approximately 1.67 micron which is restricted by the band gap of InGaAs. The semiconductor light receiving device has sensitivity to a light beam ranging from a 1.0 to 1.6 micron wavelength practically, so that the device has characteristic sufficiently covering wavelengths which are used in a conventional optical communication for a long range transmission. In recent years, demand for transmission of large amount of information such as picture images increases more and more.
A high speed network, which is capable of transmitting information to a distance ranging from several 100 m to several 10,000 meter is standardized. Long-range transmission is based on an optical communication technology for a principal line system and uses a long wavelength such as a 1.55 or 1.3 micron band. Short-range transmission uses a short wavelength of a 0.85 micron band is used.
In order to popularize the optical communication network, the semiconductor light receiving device is desired which may receive both long and short wavelength band light beams and be formed in a body. Furthermore, there is a problem that such an InGaAs/InP semiconductor light receiving device is difficult to receive a short wavelength light beam of a 0.85 micron band, as its sensitivity against a short wavelength band light beam is restricted by the 0.92 micron wavelength corresponding to the band gap of the p-type InP layer (window layer) formed on the surface of a light incident side.
In order to avoid such a problem, a semiconductor light receiving element is proposed in Japanese Patent Publication (Kokai) No. 2-231775. This element has an InP cap layer thinned to below 0.1 micron so that a light beam may be transmitted through the InP cap layer for practical use, though part of an incident light beam of a 0.7 to 0.8 micron band, for example, is absorbed in the InP cap layer. However, the semiconductor light receiving element has the problem that amount of dark current increases steeply when a bias voltage exceeds 5V.
Another semiconductor light receiving element is proposed in Japanese Patent No. 2,860,695. In fabrication of this element, a very small amount of Al (aluminum) is doped into an InP cap layer, so that the band gap of the cap layer is enlarged. Consequently, the light absorption coefficient of the InP cap layer is reduced and absorption of incident light decreases, so that the receiving light sensitivity of the element increases. There is a problem that the receiving light sensitivity is not sufficient though it inclines to increase.
To summarize the above, the semiconductor light receiving device of U.S. Pat. No. 6,521,968 is difficult to receive a short wavelength band beam below 0.9 micron. The semiconductor light receiving element of Japanese Patent Publication (Kokai) No. 2-231775 has a problem of increase of dark current. Further, the semiconductor light receiving element of Japanese Patent No. 2,860,695 has a problem that its receiving light sensitivity is insufficient.